When President Barack Obama raises his hand to take the oath of office Monday in a majestic inauguration ceremony, he’ll already be 24 hours into his second term.

He’s first taking the oath in private on Sunday, Jan. 20, the date that the Constitution says one presidential term ends and another begins. The second oath is show business, with theatrical props that include a stack of two Bibles: one used by Abraham Lincoln and one used by Martin Luther King Jr.

Remarkably, for a man elected to only two terms, Obama’s staged recitation Monday will mark the fourth time he’s sworn the presidential oath. It turns out that some oaths mean more than others, even when the words sound the same.

“For most people, they assume it’s all ceremonial, and about the trappings of the office,” Bruce G. Peabody, an associate professor of political science at Fairleigh Dickinson University, in Teaneck, N.J., said in an interview. “But very few people understand its legal significance.”

Article II of the Constitution, the part devoted to the executive branch, spells out the 35-word oath to be taken by the president:

“I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.”

The Constitution further declares that the president will take this oath “before he enter on the Execution of his Office.”

But that’s not all the Constitution says about a new president. Here’s where it can get confusing.

The 20th Amendment, ratified in 1933, adds that the “terms of the President and the Vice President shall end at noon on the 20th day of January . . . and the terms of their successors shall then begin.”

A new president’s term thus begins automatically at, say, one second past noon on Jan. 20. The new president doesn’t have to say a thing. It just happens, courtesy of the Constitution. The same Constitution, though, seemingly also requires the oath before the president exercises presidential powers.

“It’s possible to read Article II to still require the oath as a prerequisite for the president to execute the office of the presidency,” Cornell Law School professor Michael Dorf noted in an interview, “but it’s also possible to read the 20th Amendment as converting the oath into a merely ceremonial exercise.”

Careful readers of the Constitution such as Dorf and Peabody pinpoint the problems that can arise. Imagine what would happen if Obama slept through his White House alarm clock on Jan. 20 and then took a long afternoon stroll without having first taken the oath needed to “enter on the Execution of his Office.” He’s still president _ that happened at noon _ but can he order drone strikes, nominate ambassadors or exercise any other presidential power?

Or what would happen if his successor in 2017 were ill with the flu and couldn’t take the oath at noon at Jan. 20? Would Obama remain president?

The question isn’t entirely fanciful.

In January 2009, Obama and Chief Justice John Roberts Jr. verbally stumbled over each other during the Jan. 20 oath-taking. At least one word was said out of sequence, and the result proved confusing enough that the two men repeated the oath ceremony the next day at about 7:35 p.m.

“Are you ready to take the oath?” Roberts asked Obama at the second go-round, before a small White House audience.

“I am,” Obama said, “and we’re going to do it very slowly.”

Obama’s White House attorneys explained at the time that they wanted the oath repeated out of “an abundance of caution.” Peabody suggested that the caution was well-placed, noting that “a case could be made that no formal constitutional powers exist” until the correct oath is taken.

Between his botched Jan. 20, 2009, swearing-in and the second oath, though, Obama already had issued two executive orders. One, Peabody noted in his study of the issue, covered access to records and the other covered ethics requirements.

Remarkably, considering how many other challenges conservatives have leveled against Obama’s legitimacy as president, no legal case was ever brought against the two executive orders. But given the brief embarrassment that accompanied the 2009 set of oaths, it wouldn’t be surprising if both the president and the chief justice have at least mentally rehearsed how this year’s presentations will go.